1. Field of the Invention
The present invention relates to work stations or work holders of the type employed in automatic wire bonders. More particularly, the present invention relates to indexing heads for automatically positioning the bonding sites of lead frames at the bonding station of automatic wire bonders.
2. Description of the Prior Art
The present invention is an improvement of prior art work stations or work holders of the type incorporated into automatic wire bonders made by Kulicke & Soffa Industries, Inc. of Willow Grove, Pa. under model designations 1482 and 1484. Such automatic wire bonders are commercially available and have heretofore employed dedicated work holders as well as quick change work holders.
Most modern automatic work holders and work stations employ automatic positioning and automatic clamping prior to automatic wire bonding for interconnecting the pads or electrodes on semiconductor chips to the leads of a lead frame when performing the critical bonding step of making packaged semiconductor devices.
The ability of modern automatic wire bonders to precisely locate the bonding tool at a predetermined X-Y far exceeds the precision with which a lead frame can be mechanically pre-positioned in a work holder. As more dense semiconductor chips are being designed for placement on lead frames, the electrode pads of the chips as well as the conductive fingers of the lead frames to which they are to be interconnected are becoming smaller and more difficult to locate precisely.
Prior art work stations such as that shown and described in U.S. Pat. No. 4,765,531 were provided with retractable tapered pins which were inserted into index holes in the lead frames as the lead frame was being prepositioned at the bonding station. After pre-positioning the lead frame it was still necessary to determine a precise and exact position of the pads or electrodes on the semiconductor chip and the fingers of the lead frame. This type work holder which used precisor pins would and will automatically locate the index holes on the lead frame relatively to the work station, however, the electrode pads on the chip and the conductive fingers on the lead frame may not be precisely located relative to the index holes on a lead frame due to the way the lead frame was manufactured. As an example, it is erroneously assumed that lead frames made by mass production methods have edges that are both straight and parallel. Most mass produced lead frames have curved edges in the shape of a sector taken from an annular ring. It has become the practice to define this deviation or curvature in mils of camber per designated length. For example, a lead frame may have one to three mils camber per 7" of length. A lead frame with this much error will preposition the bonding sites are varying positions when the edge is used as the reference point to locate a bonding site in the bonding station. When precisor pins are inserted into indexing holes that are exactly opposite the bonding site, the effect of camber is greatly diminished, but the lack of precision in locating the indexing holes relative to the electrode pads on the semiconductor chip and the conductive fingers of the lead frame remains a problem.
Accordingly it the purpose of the present invention to provide a novel work station capable of positioning each of a plurality of bonding sites on a lead frame in a work station with an equal degree of very high precision so as to enhance the ability of an automatic wire bonder to produce high yield wire bonded semiconductor devices.